1. Field of the Invention
The present invention relates to a display device having a self-light emitting element.
2. Description of the Related Art
In recent years, a display device using a light emitting element (self-light emitting element) has been actively researched and developed. Such a display device is widely used as a display of a portable phone or a monitor of a computer by taking advantages of high image quality, thin design, lightweight and the like. In particular, such a display device provides high response speed suitable for displaying moving images, low voltage, low power consumption drive and the like, therefore, its wide applications such as a new generation portable phone and a portable information terminal (PDA) are expected.
A light emitting element is also referred to as an organic light emitting diode (OLED) and has a structure having an anode, a cathode, and a layer containing an organic compound between the anode and the cathode. The amount of current supplied to the light emitting element and a luminescence of the light emitting element are in a certain relationship. The light emitting element emits light at a luminescence according to the amount of current supplied to the layer containing the organic compound.
The major methods for displaying a multi-gray scale image by a light emitting device using a light emitting element are a voltage input method and a current input method. According to the voltage input method, a video signal inputted to a pixel is inputted to a driving element, thereby a luminescence of the light emitting element is controlled. According to the current input method, a set signal current is supplied to a light emitting element, thereby a luminescence of the light emitting element is controlled. Both methods can employ an analog driving method (analog gray scale method) and a digital driving method (digital gray scale method).
In order to prevent variations in characteristics of element driving thin film transistors which correspond to driving elements of light emitting elements in the voltage input method, a semiconductor device provided with a compensating thin film transistor between a driving power source and the element driving thin film transistor has been suggested.
[Patent Document 1]
    Japanese Patent Laid-Open No. 2002-175029
In the aforementioned patent document, variations in the threshold voltage of transistors (referred to as driving transistors) which control driving current according to gray scales is not taken into consideration. However, the inventors figured out that in the case of performing a low gray scale display in which a minute current is supplied to the light emitting element, variations in threshold voltages (Vth) become notable since a gate-source voltage (Vgs) of the driving transistor, that is a potential difference between the gate electrode and the source electrode is small.